Details emerge in twin blasts Boston Deals With Aftermath...

1of3Investigators in white jumpsuits work the crime scene on Boston's Boylston Street. Law enforcement officials said the crude devices that blew up during Monday's Boston Marathon were probably hidden inside duffel bags or backpacks.Photo: Darren McCollester / Getty Images

2of3BOSTON - APRIL 16: (EDITORS NOTE: Retransmission with alternate crop; Best quality available) Investigators in white jumpsuits work the crime scene on Boylston Street following yesterday's bomb attack at the Boston Marathon April 16, 2013 in Boston, Massachusetts. Security is tight in the City of Boston following yesterday's two bomb explosions near the finish line of the Boston Marathon, that killed three people and wounding hundreds more. (Photo by Darren McCollester/Getty Images)Photo: Getty Images

3of3BOSTON - APRIL 16: Investigators in white jumpsuits work the crime scene on Boylston Street following yesterday's bomb attack at the Boston Marathon April 16, 2013 in Boston, Massachusetts. Security is tight in the City of Boston following yesterday's two bomb explosions near the finish line of the Boston Marathon, that killed three people and wounding hundreds more. (Photo by Darren McCollester/Getty Images)Photo: Getty Images

BOSTON — The explosives that killed three people and injured more than 170 during the Boston Marathon on Monday were rudimentary devices made from ordinary kitchen pressure cookers, except they were rigged to shoot sharp bits of shrapnel into anyone within reach of their blast and maim them severely, law enforcement officials said Tuesday.

The pressure cookers were filled with nails, ball bearings and black powder, and the devices were triggered by “kitchen-type” egg timers, one official said.

The resulting explosions sent metal tearing through skin and muscle, destroying the lower limbs of some victims who had only shreds of tissue holding parts of their legs together when they arrived at the emergency room of Massachusetts General Hospital, doctors said.

Law enforcement officials said the devices were probably hidden inside dark nylon duffel bags or backpacks and left on the street or sidewalk near the finish line.

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Forensic experts said the design and components of the homemade devices were generic but that the marking “6L,” indicating a 6-liter container, could help identify a brand and manufacturer and possibly lead to information on a purchase.

New details about the explosives emerged as President Barack Obama announced that the FBI was investigating the attack as “an act of terrorism.”

But officials said they still had no suspects in custody and did not give the impression that they were close to making an arrest as they repeatedly noted that the investigation was in its infancy.

“The range of suspects and motives remains wide open,” Richard DesLauriers, the special agent in charge of the FBI's Boston office, said at a televised briefing Tuesday afternoon. And no one has claimed responsibility.

At this stage of an investigation, officials said it was not unusual for there to be no suspects.

But with the paucity of leads, DesLauriers and others pleaded with members of the public to submit any photographs or video that they may have taken at the blast site to help in the investigation.

At the briefing, DesLauriers said that someone somewhere almost certainly heard someone mention something about the marathon or the date of April 15.

“Someone knows who did this,” he said. “Cooperation from the community will play a crucial role.”

Officials said that as of Tuesday afternoon, they had received more than 2,000 tips from around the world. As marathoners left through Logan International Airport on Tuesday, security personnel reminded them of the importance of sharing their pictures with the FBI.

Counterterrorism specialists said authorities would aim to match the faces of possible suspects, using facial recognition software, against an array of databases for visas, passports and driver licenses.

“It's our intention to go through every frame of every video that we have to determine exactly who was in the area,” Edward Davis, the Boston police commissioner, said at the news briefing.

“This was probably one of the most well-photographed areas in the country yesterday.”

The scale of the attack and the crude nature of the explosives, coupled with the lack of anyone claiming to have been the perpetrator, suggested to experts that the attacker could be an individual or a small group rather than an established terrorist organization.

“This could have been a one-person job,” said Bruce Hoffman, a terrorism specialist at Georgetown University. “That makes it much harder to track. When we catch terrorists, it's usually because they're part of a conspiracy and they're communicating with one another.”

Nonetheless, a senior law enforcement official said authorities were also looking into connections between pressure cookers and al-Qaida in the Arabian Peninsula, the al-Qaida franchise in Yemen, largely because the design of the explosive device was described in a 2010 issue of the group's online English magazine, “Inspire.”

A law enforcement official said a pressure cooker in Boston “was badly damaged,” adding, “you can barely tell what it was.” But enough of it remained intact to identify it, the official said.

One brand of pressure cooker with “6L” etched on the bottom is made by the Spanish company Fagor, which, according to its website, is the fifth-largest appliance manufacturer in Europe, with factories in six countries, including Spain, China and Morocco, and subsidiaries in nearly a dozen more.

The company sells about 50,000 of the 6-liter pots in the United States every year, according to Sara de la Hera, the vice president of sales and marketing at Fagor's U.S. subsidiary.